“…Then the FD solutions Proof. By using Taylor's formula to expand (10) and (13) -(16) at the reference point 5 , 6 , 7 #, (11) at the reference point 5"!/ , 6 , 7 # , (12) at the reference point 5 , 6"!/ , 7 #, or from the discretizing procedure of (10) -(16), we could obtain the following error estimates (30). This completes the proof of Theorem 2.…”